The present invention relates to a two-circuit pressure control unit for hydraulic brake systems with two control pistons each disposed in a different one of two brake circuits. With an increasing inlet pressure, each control piston operates a valve closing against the force of a spring, and each control piston has different pressure surfaces on the opposite sides of the valve. The control unit also includes a compensation member which, by means of a surface curved in a circular manner, rests at a supporting surface of a rigid supporting member and on which the two control pistons are supported in a symmetrical arrangement with respect to the center plane of the compensation member, the supporting member preferably being displaceable by a control force.
In a known pressure control unit of this type, for instance, German Patent DE-OS No. 2,614,080, the two control pistons are arranged side by side in a parallel manner. The compensation member is a semi-circular rubber disc of elastic material. By means of the entire circumferential surface of the compensating member, the compensation member rests on a supporting surface having the same radius of curvature. On both sides of the disk, parts of the wall of the supporting member overlap the disk which has a rigid beam at the diameter surface to support the control pistons. The supporting member is held in position by a lever acted upon by a control force which is variable dependent on the axle load of the vehicle.
If the pressure at the outlet ports of the two pressure control units are different, the system which comprises the two control pistons and the compensation member will shift so as to have additional pressure fluid supplied to the lower pressure brake circuit until the pressure balance is restored. Thus, tolerance differences in the manufacture of the two control units may be compensated for up to a certain extent. Further, upon failure of one brake circuit, unhindered operation of the other brake circuit is possible. However, it has to be remembered that the compensation member is an elastic material, in particular rubber. Thus, the compensating member is subject to considerable manifestations of aging and wear. This is particularly the case because of the considerable temperature variations in motor vehicles and because of the strong forces to be absorbed upon any braking action leading to a deformation, or frictional tilting along the supporting surface. Further, there will be difficulties if the action of the two control pistons on the compensation member is not exactly symmetrical.